Anti-backlash nut assembly

ABSTRACT

An anti-backlash nut assembly is disclosed of the type which undergoes translational movement longitudinally along a screw in response to relative rotational movement between the nut and screw. The assembly includes a nut which is split into first and second portions, both of which have an internal thread complementary to the external thread of the screw. The two portions of the nut are retained in the radial direction by a coaxial cylinder. A spacer means is mounted on the retainer means intermediate opposing surfaces of the nut portions and biased against at least one of said surfaces such that any gap which may occur between the thread of the screw and the thread of the nut will be closed by movement of the spacer.

TECHNICAL FIELD

This invention is in the field of anti-backlash nut assemblies.

BACKGROUND ART

It is often desirable to drive an element in a machine along a screwwhich provides accurate positional repeatability and constant dragtorque control. Typical applications for such a device are for use indata printers and x-y tables which are used as peripheral equipment inthe computer industry.

An example of such a positioning device may be found in U.S. Pat. No.4,131,031, a previous invention of the patentees which, while it has metwith considerable commercial success, has some inherent disadvantageswhich the present invention overcomes. In the invention of the U.S. Pat.No. 4,131,031, two portions of an axially-split nut are retained in theradial direction by a spacer retaining means and a spring is used toapply longitudinal or axial biasing forces which push the internalthreads of the split nut against the flanks of the threads of thedriving screw. In order to maintain intimate contact and to avoidbacklash, it is necessary to utilize stiff springs which result in highsystem torque. The higher the system torque, the more expensive themotor needed to drive the system. Also with high system torque, a highdegree of friction is created between the threads of the backlash nutand the screw threads which creates more wear on the threads andeventually results in less stiffness in the system. As the stiffnessbecomes lower, the poorer the repeatability of the system. The apparatusof the present invention has been found to overcome the aforegoingdeficiencies in the U.S. Pat. No. 4,131,031 while at the same time,retaining some of the desirable features such as ease in manufacture andlow cost.

DISCLOSURE OF THE INVENTION

This invention discloses a new anti-backlash nut assembly designed toundergo translational movement along a threaded screw in response torelative rotational movement between the two. This anti-backlash nutassembly contains a nut having a first and second portion. Both thefirst portion and the second portion of the nut are joined together toform a complete nut which is then capable of translational movementalong the screw. Retaining means for retaining the first and secondportions around the screw are provided, and one such means comprises ahollow cylindrical spacer. Means are provided on the hollow cylindricalspacer for taking up any slack which may occur by wear of the engagedscrew threads. In a preferred embodiment of the invention, the means fortaking up the slack consists of an internally threaded spacer which ismounted on threads provided on the retaining means and which istorsionally biased by a torsion spring. The spacer is forced to rotateby the torsion spring whenever any gap attempts to develop between thetwo opposing portions of the nut.

It may thus be seen that the anti-backlash nut assembly of thisinvention has several outstanding advantages. One of the main advantagesis that in the apparatus of the present invention, unlike that of theU.S. Pat. No. 4,131,031, we are not relying on the force of the springfor system torque. Instead, in the apparatus of the present invention,there can be negligible system torque. In other words, in the "031"patent, a spring is used to establish a pre-load force which holds thenut threads against the screw threads. In the present invention, no suchpre-load force is required. At the same time, axial stiffness also doesnot depend on the stiffness of the spring but is instead dependent onthe fact that intimate contact between the flanks of the screw threadsis maintained as the spacer means rotates on the retaining means andmaintains intimate contact between the threads of the opposing nutmembers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation of the disassembled components of ananti-backlash nut assembly of this invention;

FIG. 2 is a perspective view illustrating an anti-backlash nut assemblyaccording to this invention;

FIG. 3 is a perspective view of the retaining means of this invention;

FIG. 4 is a perspective view of the spacing means of this invention;

FIG. 4a is a perspective view of an alternate embodiment of theinvention using a spacing means 28(a) and an additional elastomericspacer 28(b).

FIGS. 5 and 6 illustrate in schematic form the operation of theinvention;

FIG. 7 is a cross-sectional view of another embodiment of the invention;

FIG. 8 is a perspective view of a further alternate embodiment of asplit nut assembly of the invention;

FIG. 9 is a sectional view of elements assembly of FIG. 7 taken alonglines 8-9.

BEST MODE OF CARRYING OUT THE INVENTION

This invention can be further described by referring to the Figures inmore detail.

In FIGS. 1-4, one embodiment of an anti-backlash nut assembly isillustrated having a screw 10 and a nut assembly 12. The uniqueconstruction of nut assembly 12 can be seen clearly in FIGS. 1-4. Thereare two opposing nut portions 14 and 22. The first portion 14 has atubular section 16 with an enlarged outside diameter and a through holefor screw 10 and a face plate 18 to which apparatus such as a printercarriage (not shown) may be bolted. Face plate 18 also has a throughhole for screw 10. Screw 10 is normally driven by a step motor (also notshown). In some applications, the printer will print in both directionsof travel of the nut assembly 12. In such applications, it is of utmostimportance that minimum backlash occur in the gearing between the screw10 and the carriage.

The first portion 14 of the nut forms approximately one-half of acylindrical nut having internal threads 15 complementary to the externalthreads on screw 10. The second half 22 of the axially-split nut isprovided with a raised shoulder 21 with an increased outside diameterbut also having a through hole for screw 10. The second portion 22 alsohas internal threads 20 complementary to the external threads on screw10. So far, the portions described are substantially identical to theelements shown in the patentees U.S. Pat. No. 4,131,031.

First portion 14 and second portion 22 are retained in an alignedposition about screw 10 by retainer means 24. Retainer means 24 has aninternal cylindrical shape which is coaxially disposed about threadedportions 20 and 15 of nut portions 22 and 14 to retain the opposinginternal thread surfaces of nut portions 22 and 14 in alignment with thethreads on screw 10.

An external thread 25 is provided as shown on the right hand portion ofretaining means 24. Coaxially mounted on retaining means 24 is a torsionspring 26. The tangs 30 and 32 of which are engaged respectively inopenings 23 and 29. The opening 29 is provided at one end (the right endside) of retainer means 24 and as more clearly shown in FIG. 4 opening23 is located on the inside, or left side, surface of spacer means 28.

Spacer means 28 consists of a cylindrical spacer as shown in FIG. 4having internal threads 27 which mate with the threads on retainer means24. As can be seen from the schematic representation of FIGS. 5 and 6,the operation of the rotatably mounted spacer is as follows. The threadon the screw 10 or nut assembly will wear through use tending to createa space between opposing nut portions. Assume the wear is equal to Δx.There the space between nut halves would increase from A to B by adistance of 2Δx. However, in accordance with this invention, as thisoccurs, the spacer means 28 which is torsionally biased by spring 26 isnow free to rotate about the thread on retaining means 24 and thus takeup the gap 2Δx left by nut wear.

The internal thread on spacer 28 is designed to have a sufficiently fineenough thread so as not to be contrarotated once advanced. The assemblythereby creates an extremely stiff anti-backlash of U.S. Pat. No.4,131,031. At the same time, the torsion spring force required to rotatethe spacer means 28 need only be enough to rotate the spacerlongitudinally along the retainer 24 thereby imparting very little forcebetween opposing nut halves. We thus have a very low torque system withextremely high system stiffness.

The various components of the axially-split anti-backlash nut assemblycan be fabricated from a variety of materials. The first and secondportions (respectively 14 and 22) of the nut could, for example, befabricated from metal such as steel or cast bronze, or from anythermoplastic moldable polymer composition. It is preferable to moldcomponents which contact the screw from low-friction self-lubricatingpolymer compositions which have outstanding wear characteristics. Othercomponents could be similarly made from metal or plastics.

It may also be desirable in some instances to provide a slightlyelastomeric material such as a rubber washer 28b (as shown in FIG. 4A)itermediate the spacer 28a and the nut portion 22. The elastomericspacer 28b would make the system slightly less stiff yet still allow forslight shaft irregularities frequently encountered in rolled formed leadscrews 10, thus maintaining torque consistency over shaft length.

Another embodiment of the invention having a split nut arrangementwhereby the nuts will move axially yet not rotate with respect to eachother is shown in FIGS. 7 and 9 in which a bar 38 interfits with groovesin opposing nut portions 40 and 42 so as to permit axial (longitudinal)movement for a distance x of the nut portions but which preventsrotational or torsional movement of the opposing nut portions. A torsionspring 46 is located between nut portion 42 and spacer means 48. Therespective tangs of spring 46 are inserted in openings 49 and 43provided in retainer 44 and spacer means 48. Spacer 48 is rotatablymounted on external threads provided on nut portion 40. The spacer, aspreviously described, will take up any slack which may occur because ofwear between the screw threads on screw 10 and internal threads(notshown) on nut portions 40 and 42.

FIG. 8 illustrates yet another embodiment of the invention wherein theopposing nut portions 52 and 50 consist of internally threadedinterfitting members one of which has a squared external member 54 whichfits inside the squared opening 56 in nut portion 50 whereby nut portion52 is restrained from rotational movement with respect to nut portion 50yet axial movement is provided between the two; which axial movement istaken up by the operation of torsion spring 58 and spacer means 28 aswas described in connection with the previously described embodimentsshown in FIGS. 1-7.

EQUIVALENTS

Those skilled in the art will recognize, or be able to ascertain, usingno more than routine experimentation, many equivalents to specificcomponents, elements, and steps described herein. For example, theretainer means 24 could be integrally molded or joined with left handnut portion 14. Such equivalents are intended to be covered by thefollowing appended claims.

I claim:
 1. An assembly, comprising in combination:a. a first nutportion having an internal thread; b. a second nut portion having aninternal thread; c. retaining means for retaining said first nut portionand said second nut portion in a fixed non-radially rotatable alignedposition with respect to each other on a threaded screw; and, d. spacermeans between said first and second nut portions and rotatably mountedon said retaining means; and, e. biasing means causing the spacer meansto react to any slack between opposing nut portions and axially rotateagainst at least one surface of one nut portion.
 2. A nut assembly ofclaim 1 wherein the biasing means is a torsion spring having a tang oneach end of the spring, one of which is inserted in said spacer meansand the other tang is inserted in said retaining means.
 3. Ananti-backlash nut assembly of claim 1 wherein the nut portions compriseleft and right axially-split nut portions.
 4. An anti-backlash nutassembly of claim 1 wherein an elastomeric washer is providedintermediate the spacer means and the .[.retaining means.]. .Iadd.saidone surface of one nut portion. .Iaddend.
 5. An anti-backlash nut andscrew assembly in which the nut is designed to undergo translationalmovement along the screw, said screw having an external thread thereon,the movement of the nut being in response to relative rotationalmovement between the nut assembly and screw, comprising, incombination:a. a first portion of said nut having an internal threadcomplementary to the external thread of said screw; b. a second portionof said nut, also having an internal thread complementary to theexternal thread of said screw; c. retaining means for retaining saidfirst portion and said second portion of the nut in a non-radiallyrotatable aligned position around said screw to enable the internalthreads of said nut portions to engage with the external threads of saidscrew; d. spacer means adapted to move with respect to the first andsecond nut portions; and, e. biasing means for causing the spacer meansto rotate on the retaining means about the screw axis and abut a surfaceof one of the nut portions and take up any slack between opposing nutportions which may occur from wear of the screw surfaces of the nut orscrew.
 6. An anti-backlash nut assembly of claim 5 wherein the spacermeans has an internal thread which mates with external threads on saidretaining means.
 7. An anti-backlash nut assembly of claim 5 whereinsaid first and said second portions are restrained from torsionalmovement by a bar member interfitted into grooves on opposing portionsof the nut.
 8. An anti-backlash nut assembly of claim 5 in which thefirst portion of the nut has a non-circular outer surface whichinterfits with a corresponding non-circular inner surface of the secondnut portion.